Ink jet technology with aqueous inks is very well understood when jetting fluids of 1-5 mPa-sec or less. New applications using non-aqueous fluids as well as aqueous fluids with viscosities up to 100 mPa-sec present new challenges both for steady state jetting of fluids and during an initial fluid jetting start after a period of non-ejection of fluid. Fluids will often fail to eject from micro-fluid jet heads when the jet head ejection nozzles or jet heads are left uncapped for relatively short periods of time without a wiping or maintenance step built into the ejection sequence despite previous ejection from the jet heads. The foregoing ejection problem may be aggravated by increased viscosity of the fluid being jetted. Accordingly, the initial ejection of higher viscosity fluids from an uncapped ejection head is a challenge to the use of relatively high viscosity fluids in a micro-fluidic ejection device. By “high viscosity” is meant viscosities in the range of from about 20 to about 100 mPa-sec or higher at about 22° C. Furthermore, such high viscosity fluids are often required to be used in environments of temperature and humidity that are outside traditional limits of temperature and humidity used by ink jet printer and print head manufactures. Applications for jetting high viscosity fluids may include, but are not limited to, high viscosity inks, adhesives, adhesive components, solid-to liquid phase change compositions, pharmaceuticals, aroma enhancing compounds, and the like. Accordingly, there is a need for micro-fluid ejection heads that are adapted for use with relatively high viscosity fluids.
Embodiments of the disclosure provide methods for ejecting fluids having a viscosity ranging from about 20 mPa-sec to about 100 mPa-sec at 22° C. from a micro-fluid ejection head. The methods include the steps of applying a heat signal to the ejection head for a first period of time to heat the ejection head to a first temperature that is about 20° C. above a steady state fluid ejection temperature for continuous or intermittent fluid ejection from the ejection head; and subsequently, applying a firing signal to ejection heaters on the ejection head during which fluid ejection from the ejection head occurs.
In one embodiment, the method for ejecting a high viscosity fluid for a first time from a newly filled micro-fluid ejection head or after an ejection head idle period of 60 minutes or more includes the steps of pre-heating the ejection head to a temperature ranging from about 60° C. to about 100° C. and maintaining the temperature for a first period of time ranging from about 30 to about 60 seconds by applying a pre-heat signal to one or more substrate heaters on the ejection head; applying a fluid ejection signal to the ejection head subsequent to the pre-heat signal to eject drooling fluid from the ejection head, wherein the fluid ejection signal has a pre-fire pulse of 250 to 350 nanoseconds (nsec), a dead time of 1200 nsec, and a firing pulse 750 to 1000 nsec; subsequently, applying a heat signal to the one or more substrate heaters on the ejection head for a period of time ranging from about 3 to about 6 seconds to heat the ejection head to a temperature that is about 20° C. above a steady state fluid ejection temperature for continuous or intermittent fluid ejection from the ejection head; and subsequently, applying a firing signal to the ejection heaters on the ejection head during which steady state fluid ejection from the ejection head occurs.
In another embodiment, a method for ejecting a solid material having a melting point of from about 20° to about 30° C. from a micro-fluid ejection head is provided. The method includes the steps of heating the solid material in a container for the material that is adjacent to the ejection head to a temperature sufficient to provide a flowing liquid having a viscosity of from about 20 to about 100 mPa-sec; applying a heat signal to one or more substrate heaters on the ejection head for a first period of time to heat the ejection head to a first temperature that is about 20° C. above a steady state fluid ejection temperature for continuous or intermittent fluid ejection from the ejection head; and subsequently, applying a firing signal to ejection heaters on the ejection head during which fluid ejection from the ejection head occurs, wherein the firing signal has a pre-heat pulse of 200 to about 300 nanoseconds (nsec) a dead time of about 1200 nsec and a firing pulse of 700 to about 950 nsec.
The foregoing methods are particularly suitable for the initial ejection of fluids having high viscosity from a thermal fluid ejection head that is being used for the first time, that has been initially filled with high viscosity fluid, or that has cooled down below about 30° C. due to non-use of the ejection head. The fluids may be liquid below about 30° C. or may be materials that go through a phase change from solid to liquid. A modified procedure, described in more detail below, may be used when the ejection head is at a temperature ranging from above about 30° C. to below about 50° C. An advantage of the disclosed methods is that the procedure is effective to initiate ejection of a high viscosity fluid from a micro-fluid ejection head without the need for thermal ejection head wipers or elaborate maintenance procedures, such as the use of suction to clear any fluid plugs in nozzles and flow feature of the ejection head.